Both heat and cold evoke thermosensation, which, if of sufficient intensity, may elicit feelings of pain. Temperature detection in mammals is a critical function in the maintenance of thermal homeostasis and protection from potentially injurious temperature extremes. Considerable efforts have been put into elucidating the biochemical mechanisms involved in the detection, transduction and transmission of hot and cold sensations in neuronal tissues. Thermal stimuli activate specialized receptors located on sensory neurons, such as those deriving from the dorsal root ganglion (DRG) and the trigeminal ganglion (TG). When these stimuli are in the noxious range (i.e., very hot or cold), they activate a certain subset of thermo-sensitive receptors on a sub-population of sensory neurons called nociceptors (pain-sensing neurons). Upon activation, the thermo-sensitive receptors (e.g., ion channels) transduce the noxious stimulus into an electrical signal that is propagated along the sensory neuron to the spinal cord, where it is relayed to the brain, ultimately leading to the perception of pain. Accordingly, these thermo-sensitive receptors represent highly promising targets for developing drugs for the treatment of various painful conditions, particularly those in which thermal hypersensitivity and/or thermally evoked pain are present.
Several thermo-sensitive receptors have been implicated in sensing temperature changes. Currently, six members from the transient receptor potential (TRP) ion channel family have been proposed to serve as thermal sensors of temperatures ranging from noxious heat to noxious cold and thus are referred to as thermo-TRPs. The thermo-TRPs, which are non-selective cation channels, are also sensitive to chemical stimuli, such as capsaicin, menthol mustard oil, etc., which are known to produce a burning or cooling sensation. Importantly, all of the thermo-TRPs are expressed in nociceptive sensory neurons, either Aδ- or C-fibers, wherein thermo-, mechano- and chemo-sensory transduction take place. TRPV1, the most extensively characterized member of this family, is activated by moderate heat (˜43° C.), capsaicin, protons and certain lipidic mediators, such as anandamide (Caterina, M. J., et al. 1997, Nature 389 (6653), 816-824; Tominaga, et al. 1998, Neuron 21(3), 531-543) Moreover, numerous studies using a variety of experimental approaches have implicated TRPV1 in the transduction of pain signals in animal models of hyperalgesia (Caterina, M. J., et al. 2000, Science 288(5464), 306-313; Davis, J. B., et al. 2000 Nature 405(6783), 183-187); hence, this TRP has been aggressively pursued as a target for the alleviation of certain types of pain.
The transient receptor potential V2 (TRPV2) protein (previously called and also known as vanilloid receptor-like 1 or VRL-1) is a member of the transient receptor potential (TRP) superfamily, which has been implicated in mediating diverse cellular functions, including the transduction of thermal, chemical and mechanical stimuli. TRPV2 is closely related to TRPV1 and was identified by homologous cloning from rat and human (Caterina et al., 1999, Nature 398(6726), 436-441) as well as mouse (Kanzaki, et al. 1999, Nat Cell Biol 1(3), 165-170). Functional studies have revealed that rat TRPV2 responds to noxious heat, with an activation threshold above 52° C. (Caterina, Rosen et al. 1999) and that mouse TRPV2 responds to changes in osmolarity or to membrane stretch (Muraki, et al. 2003, Circ Res 93(9), 829-838). The hypothesis that TRPV2 is an endogenous sensor of noxious heat and mechanical stretch is further supported by findings that it is expressed in medium to large diameter Aδ mechano- and thermo-sensory neurons in the dorsal root ganglion (DRG) (Caterina et al., 1999, Nature 398(6726), 436-441).
Unlike TRPV1, however, no selective activators of TRPV2 have been reported. Recently, a novel class of selective agonists, such as Δ9-tetrahydrocannabinol (Δ9-THC) as well as other related cannabinoids, that activate human, rat and mouse TRPV2 have been identified. (See U.S. application Ser. No. 11/589,340, entitled COMPOSITIONS AND METHODS FOR IDENTIFYING MODULATORS OF TRPV2 to Qin et al., filed Oct. 30, 2006, now U.S. Pat. No. 7,575,882, issued Aug. 18, 2009).
There is a need to identify additional thermo-sensitive receptors, as they are potential targets for the treatment of pain. There is also a need to identify thermo-sensitive receptors in different species, as they can be used as model systems to investigate the effects of test compounds. Particularly, there is a need for systems that can be used to test compounds that potentially increase or decrease the activity of a thermo-sensitive receptor responding to noxious thermal stimuli, including noxious heat. Identification and testing of such compounds would enable the treatment of various disorders associated with neuropathic pain, inflammatory pain, various chronic pains or for uses in other conditions, such as the inflammatory immune response, febrile seizures and general epilepsy and/or conditions in which thermal hypersensitivity or thermally evoked pain are present as well as other human diseases related to these thermo-sensitive receptors.